In metal cutting, particularly in lathing of ductile materials, continuous chips are usually generated. Such continuous chips cause serious difficulties in operational safety, chip disposal and quality control of the machined workpiece surface. More seriously, continuous chips can cause unpredictable jams in the cutting process, which prevent automation of machining systems. Therefore, in metal cutting it is strongly desired to have the chips generated in discontinuous forms. Chip breakers for metal cutting processes are devices used for breaking chips into small pieces. Two types of chip breakers have been proposed in the prior art--obstruction devices separate from a tool insert, and combinations of grooves and obstacles formed on the rake face of the tool insert. During metal cutting processes, such chip breakers work to bend the chip after it has been generated. These chip breakers do not make sure that the chip is broken during cutting and chip breaking can be achieved only in some machining conditions. It is very difficult to predict whether the chip will be broken by these chip breakers from a set of given machining conditions, that is, when these chip breakers are used in metal cutting, it is uncertain that the chip will break. Further, when chips are broken by such chip breakers, the chips usually fly in many directions and these still cause difficulties in operational safety, control of machined workpiece surface quality and chip disposal. Furthermore, some of these chip breakers weaken the tool cutting edge and consequently shorten the tool life. Typical examples of existing chip breakers may be seen in patent specifications numbers U.S. Pat. No. 5,193,947, SU 1704939, WO 9427769 and WO 9511102.